Add more tests for IPv6 Uri delimiters

src/libraries/System.Private.Uri/src/System/Uri.cs

@@ -3888,7 +3888,7 @@ private unsafe int CheckAuthorityHelper(char* pString, int idx, int length,
             if (ch == '[' && syntax.InFact(UriSyntaxFlags.AllowIPv6Host) &&
                 IPv6AddressHelper.IsValid(pString, start + 1, ref end))
             {
-                if (end < length && pString[end] is not (':' or '/' or '?' or '#'))
+                if (end < length && pString[end] is not (':' or '/' or '\\' or '?' or '#'))
                 {
                     // A valid IPv6 address wasn't followed by a valid delimiter (e.g. http://[::]extra).
                     flags |= Flags.UnknownHostType;

src/libraries/System.Private.Uri/tests/FunctionalTests/UriIpHostTest.cs

@@ -378,20 +378,61 @@ private void ParseIPv6Address(string ipv6String, string expected)
             // CheckHostName
             Assert.Equal(UriHostNameType.IPv6, Uri.CheckHostName(ipv6String));
 
-            // IP followed by extra chars without a valid delimiter is invalid
-            Assert.False(Uri.TryCreate($"http://[{ipv6String}]extra", UriKind.Absolute, out _));
-            Assert.False(Uri.TryCreate($"http://[{ipv6String}] extra", UriKind.Absolute, out _));
-            Assert.False(Uri.TryCreate($"http://[{ipv6String}]\\extra", UriKind.Absolute, out _));
-
-            CheckProperties(new Uri($"http://[{ipv6String}] "));
-            CheckProperties(new Uri($"http://[{ipv6String}]/extra"), path: "/extra");
-            CheckProperties(new Uri($"http://[{ipv6String}]:"));
-            CheckProperties(new Uri($"http://[{ipv6String}]:123"), 123);
-            CheckProperties(new Uri($"http://[{ipv6String}]?extra"), query: "?extra");
-            CheckProperties(new Uri($"http://[{ipv6String}]#extra"), fragment: "#extra");
-
-            void CheckProperties(Uri uri, int port = 80, string path = "/", string query = "", string fragment = "")
+            // Various combinations of different schemes and suffixes
+            TestSuffixes("http://", 80);
+            TestSuffixes("custom-scheme://", -1);
+            TestSuffixes("file://", -1, backslashIsAllowed: true); // UNC path
+            TestSuffixes("file:////", -1, backslashIsAllowed: true); // UNC path
+            TestSuffixes("\\\\", -1, backslashIsAllowed: true, implicitFile: true); // UNC path
+
+            if (!UriParser.IsKnownScheme("parse-ip-file-based-scheme"))
             {
+                UriParser.Register(new FileStyleUriParser(), "parse-ip-file-based-scheme", -1);
+                UriParser.Register(new HttpStyleUriParser(), "parse-ip-http-based-scheme", 12345);
+            }
+
+            TestSuffixes("parse-ip-file-based-scheme://", -1, backslashIsAllowed: true);
+            TestSuffixes("parse-ip-http-based-scheme://", 12345);
+
+            void TestSuffixes(string prefix, int port, bool backslashIsAllowed = false, bool implicitFile = false)
+            {
+                // IP followed by extra chars without a valid delimiter is invalid
+                Assert.False(Uri.TryCreate($"{prefix}[{ipv6String}]extra", UriKind.Absolute, out _));
+                Assert.False(Uri.TryCreate($"{prefix}[{ipv6String}] extra", UriKind.Absolute, out _));
+
+                // Some schemes (particularly file) allow backslashes to separate the host and path
+                if (backslashIsAllowed)
+                {
+                    CheckProperties(new Uri($"{prefix}[{ipv6String}]\\extra"), implicitFile, port, path: "/extra");
+                }
+                else
+                {
+                    Assert.False(Uri.TryCreate($"{prefix}[{ipv6String}]\\extra", UriKind.Absolute, out _));
+                }
+
+                // Regular delimiters like ? and # are allowed
+                CheckProperties(new Uri($"{prefix}[{ipv6String}] "), implicitFile, port);
+                CheckProperties(new Uri($"{prefix}[{ipv6String}]/extra"), implicitFile, port, path: "/extra");
+                CheckProperties(new Uri($"{prefix}[{ipv6String}]?extra"), implicitFile, port, query: "?extra");
+                CheckProperties(new Uri($"{prefix}[{ipv6String}]#extra"), implicitFile, port, fragment: "#extra");
+
+                if (port >= 0)
+                {
+                    CheckProperties(new Uri($"{prefix}[{ipv6String}]:"), implicitFile, port);
+                    CheckProperties(new Uri($"{prefix}[{ipv6String}]:123"), implicitFile, 123);
+                }
+            }
+
+            void CheckProperties(Uri uri, bool implicitFile = false, int port = 80, string path = "/", string query = "", string fragment = "")
+            {
+                if (implicitFile)
+                {
+                    // Implicit file paths don't support queries/fragments, everything is treated as the path
+                    path += Uri.EscapeDataString(query) + Uri.EscapeDataString(fragment);
+                    query = "";
+                    fragment = "";
+                }
+
                 Assert.Equal(UriHostNameType.IPv6, uri.HostNameType);
                 Assert.Equal(expectedResultWithBrackets, uri.Host);
                 Assert.Equal(expected, uri.DnsSafeHost);